The present invention relates to an improved process for producing a magnetic head of a type used in a video tape recorder, digital audio tape recorder or the like and mounted at a selected location on the periphery of a rotated drum with which magnetic tape makes helical contact for achieving helical-scan inclined-azimuth recording.
Recent technological advances toward higher packing densities in magnetic recording have been remarkable. This has caused a growing need to reduce the track width produced by magnetic heads in both recording and playback modes, as well as the width of the magnetic gap. Taking 8 mm video cameras and digital audio tape recorders as examples, the magnetic head produces a track width of as small as about 10 microns and a gap width of 0.2 to 0.3 microns.
The magnetic gap of such a head and its nearby area are shown enlarged in FIG. 1. Shown by 1a and 1b are core halves that combine together to form a magnetic circuit in the head, 2 is a glass material that is filled in track width controlling grooves formed in the mating surfaces of core halves 1a and 1b and which serves to join the two core halves together, and 3 is nonmagnetic thin film that forms a magnetic gap, or a working gap, between the core halves 1a and 1b.
Magnetic heads having such a construction are mounted at two or more locations on the periphery of a cylindrical drum which is rotated as magnetic tape makes helical contact with the drum, thereby achieving helical recording and reproduction on the tape.
In helical-scan inclined-azimuth recording of the type described above, two or more magnetic heads having different azimuthal angles are employed, and the track width of each head is set at a value slightly larger than the pitch of adjacent tracks formed on the tape. As shown in FIG. 2, as the tape is driven across the rotating drum, the two magnetic heads record information at different azimuthal angles, one head producing a recording pattern 6a and the other head producing a pattern 6b . This is described below more specifically with respect to the recording mode (FIG. 3) and the playback mode (FIG. 4). In FIG. 3 an arrow A indicates the direction of tape transport and an arrow B designates the direction in which the head rotates. Of the two magnetic heads, the first head produces a recording pattern 16a at a first azimuthal angle. Since the tape is running during the production of pattern 16a, the second magnetic head touches an area of the tape that overlaps the pattern 16a so as to erase part of the latter, thereby producing a recording pattern 16b that overlaps the pattern 16a but which has a different azimuthal angle. The recording patterns produced by scanning with the first and second heads in the manner described above are designated by 6a and 6b in FIG. 2.
In the playback mode (FIG. 4), the first head having a track width C, which is greater than the width of the track recorded on the magnetic tape, scans the tape in such a way that it bridges adjacent tracks recorded by the second head. Since the two magnetic heads have different azimuthal angles, no information is reproduced from the tracks recorded by the second head and only the tracks recorded by the first head are reproduced.
A magnetic head used to perform such recording/reproducing functions is shown diagrammatically in FIG. 5. In order to fabricate such a magnetic head, core halves 1a an 1b are machined to form a plurality of track width controlling grooves spaced apart by a distance corresponding to the track width. Thereafter, a magnetic-gap-forming nonmagnetic thin film is deposited on the mating surfaces of the core halves, which are joined together in such a way that the opposite track width controlling grooves are in registry with each other. A glass material 2 is then filled in each groove and the core halves 1a and 1b are bonded together to form a single core block. The block is sliced into discrete magnetic heads by cutting along lines spaced apart by a distance equivalent to the width indicated by 5 in FIG. 5. The gap and the nearby area of one of such individual magnetic heads is shown enlarged in FIG. 1.
Fabrication of a magnetic head as illustrated in FIG. 5 has the problem that it is very difficult to achieve complete registry between opposite track width controlling grooves and misalignment often occurs in the magnetic heads produced by slicing the core block as illustrated in FIG. 1. In this situation, the width of the magnetic gap that is effectively used for recording and playback purposes is limited to the area indicated by 4 in FIG. 1. Such misalignment presents a serious problem in a magnetic head having a small track of only about 10 microns. In order to avoid such problems, very close tolerances are required in machining and joining the block halves 1a and 1b.
One approach that has been taken to prevent the occurrence of misalignment between opposite grooves formed in the two core halves 1a and 1b is to cut the groove in such a way that the magnetic gap defined by the grooves cut in the core half la has a different width from the gap defined by the grooves cut in the other core half 1b. The core halves so worked are subsequently joined together. The track width of the magnetic head fabricated by this method is defined by the narrower gap in the core half 1a.
However, the following problem occurs if this magnetic head is used in helical-scan inclined-azimuth recording. As already explained with reference to FIGS. 2 and 3, information is overwritten on the magnetic tape so as to produce a track width greater than the pitch of adjacent tracks on the tape. In this case, however, the edge of the individual recorded tracks is determined by the end of the magnetic head gap from which magnetic flux leaks. In the case of the magnetic gap shown in FIG. 6, the magnetic field produced by this leakage flux is small and each of the tracks formed on the magnetic tape will produce a sharp magnetic end, producing recording patterns as shown in FIG. 2. On the other hand, if recording is made with a magnetic head as shown in FIG. 7, a large flux leaks from the end of the magnetic gap to produce an increased leakage magnetic field. As a result, the magnetic gap end becomes indistinct and extends beyond the mechanical gap end to cause undesired recording effect.
In addition, the region 7 that determines recording magnetization in the vicinity of the gap becomes curved as shown in FIG. 8 and the recorded track pattern has the relationship shown in FIG. 9 with respect to the magnetic head depicted in FIG. 10. FIG. 10 shows the track pattern produced by this magnetic head. Reference numeral 8 in Fig. 11 indicates the area where information is recorded at an azimuthal angle different from that of the magnetic head used to achieve this recording. This area effectively reduces the track width of the playback head, thereby reducing the reproduction output. This area, which does not contribute to the reproduction output, is a significant factor in 8 mm video cameras and audio digital tape recorders which have a small track width. Thus, the reproduction output is inevitably somewhat reduced.